Michael Batty: The Emergence of Cities: Complexity and Urban Dynamics
Abstract:This paper presents an approach to urban dynamics that generalizes the traditional rank-size model first popularized by Zipf (1949). It argues that we need to define the rate at which new cities emerge and old cities disappear within the apparent macro stability posed by Zipf s Law. We illustrate this with a reworking and extension of Zipf s analysis of the US urban system, taking his analysis from 1790 to 1930 forward to the year 2000. In doing so, we introduce a variety of devices to detect urban change based on traces through the rank-size phase space, trajectories using a rank-time clock, and the definition of urban half-lives. We set this analysis within the wider context of stochastic simulation that is currently dominating discussion of scaling processes such as these.

Michael Batty: Agents, Cells and Cities: New Representational Models for Simulating Multi-Scale Urban Dynamics
Abstract:New forms of representation at a fine spatial scale, where units of space are conceived as cells and populations as individual agents, are currently changing the way we are able to simulate the evolution of cities and related systems. In this paper, we review progress to date in this field. We show how these new approaches are consistent with traditional urban models that have gone before with the emphasis no longer being on spatial interaction but on the dynamics of development and local movement. We first introduce a generic structure for urban simulation based on ideas about spatial evolution as reaction and diffusion, and then show how problems conceived in terms of cells, or agents, or both enable new implementations of this generic model. We sketch the rudiments of cellular automata (CA) which emphasises rules of development, and agent-based models which focus on how agents respond to attributes of their environment often encoded in cellular landscapes. We develop various exemplars based on residential location to impress the way these approaches work. Three applications are then presented at very different spatial scales: first pedestrian movement at the building scale, then the evolution of systems of cities at a country scale, and finally urban growth at the city scale. In developing these approaches, we show how cellular and agent-based models have the potential for explicitly incorporating spatial interaction and transportation which is their current weakness. We conclude with proposals that formal policy analysis in this domain should always be informed by more than one approach.

Michael Batty: Agent-Based Pedestrian Modelling
Abstract:When the focus of interest in geographical systems is at the very fine scale, at the level of streets and buildings for example, movement becomes central to simulations of how spatial activities are used and develop. Recent advances in computing power and the acquisition of fine scale digital data now mean that we are able to attempt to understand and predict such phenomena with the focus in spatial modelling changing to dynamic simulations of the individual and collective behaviour of individual decision-making at such scales. In this Chapter, we develop ideas about how such phenomena can be modelled showing first how randomness and geometry are all important to local movement and how ordered spatial structures emerge from such actions. We focus on developing these ideas for pedestrians showing how random walks constrained by geometry but aided by what agents can see, determine how individuals respond to locational patterns. We illustrate these ideas with three types of example: first for local scale street scenes where congestion and flocking is all important, second for coarser scale shopping centres such as malls where economic preference interferes much more with local geometry, and finally for semi-organised street festivals where management and control by police and related authorities is integral to the way crowds move.

Joana Barros, Fabiano Sobreira: City of Slums: self-organisation across scales
Introduction:The city is certainly a fine example of a complex system, where the parts can only be understood through the whole, and the whole is more than the simple sum of the parts. In the present paper we explore the idea that some of these parts are themselves complex systems and the interrelation between complex subsystems with the overall system is a necessary issue to the understanding of the urban complex system. Spontaneous settlements are clear examples of complex subsystems within a complex urban system. Their morphological characteristics combined with their development process are traditionally understood as chaotic and unorganised. And so are Third World cities, traditionally known for their inherent chaotic and discontinuous spatial patterns and rapid and unorganised development process. The paper consists in a brief theoretical analysis developed on the interrelationship between two urban processes across scales: the local process of formation of inner-city squatter settlements and the global process of urban growth. What is the role that spontaneous settlements play in the global dynamics of the city? We explore this issue by analysing experiments of City-of-slums , an agent-based model that focuses on the process of consolidation of inner-city squatter settlements within a peripherisation process. The paper also includes two previous studies on these topics where the dynamics of these two urban processes are examined as two isolated complex systems and an analysis of the morphological fragmentation of the distribution of spontaneous settlements within the overall city and within the spontaneous settlements themselves. Based on these analyses, we conclude with a brief discussion on the role of self-organisation in the socio-spatial dynamics of Third World cities.